A native of Dunboyne, County Meath,[1] Lonergan left his schoolboy club Cherry Orchard to join the academy of St Patrick's Athletic in March 2019, where he initially played with the U15 side, before progressing to the U17 and U19 teams over the following 3 seasons.
[4] Lonergan's senior debut for the club came on 23 July 2021, when he replaced Billy King from the bench for the final 10 minutes of a 6–0 win over Bray Wanderers in the FAI Cup.
[16] He scored the winning goal of the game in the 2022 Promotion/Relegation Playoff Final on 11 November 2022 as his side defeated Waterford 1–0 at Richmond Park to confirm their League of Ireland Premier Division status for 2023.
[17] His form for UCD drew interest from all of the top 4 clubs, with Dundalk and St Patrick's Athletic offering him contracts, while Derry City and Shamrock Rovers were reportedly monitoring his situation.
[20] On 12 July 2023, Lonergan made his first appearance in European competition when he came off the bench for the final 9 minutes of a 2–1 loss against F91 Dudelange in the first qualifying round of the UEFA Europa Conference League at the Stade Jos Nosbaum, in which his cross was unsuccessfully cleared by a defender, allowing Mark Doyle to pull a goal back for his side.
[21] On 15 September 2023, Lonergan scored an 85th minute winner in a 2–1 win away to Finn Harps in the FAI Cup, to earn his side a place in the Semi Final.
[24] On 1 February 2024, it was announced that Lonergan had signed for EFL League One club Fleetwood Town on a two-and-a-half-year contract for an undisclosed fee.